Improve type-widening and add tests

[rsokl]

builds performs type-widening when users specify targeted configs as default values. This prevents common friction with Hydra's validation system; see #84

This PR fixes a case where the type-widening behavior was too aggressive: A targeted config type should not be broadened to Any since this is a valid type that can be leveraged by Hydra's runtime validation. Thus this PR fixes this, so that dataclass-types are retained and thus can be leveraged by Hydra for validation.

Before:

from hydra_zen import builds, instantiate

BuildsInt = builds(int)

def f_with_dataclass_annotation(x: BuildsInt = BuildsInt()):
    return x

>>> bad_value = builds(str)()  # not an instance of `BuildsInt`
>>> instantiate(builds(f_with_dataclass_annotation, x=bad_value))  # should raise validation error
''

After

>>> bad_value = builds(str)()
>>> instantiate(builds(f_with_dataclass_annotation, x=bad_value))
---------------------------------------------------------------------------
ValidationError   
ValidationError: Invalid type assigned: Builds_str is not a subclass of Builds_int. value: Builds_str(_target_='builtins.str')
    full_key: x
    object_type=None

>>> ok_value = builds(str, builds_bases=(BuildsInt,))()  # inherits from `BuildsInt`: type-checking passes
>>> instantiate(builds(f_with_dataclass_annotation, x=ok_value))
''

This PR also ensures that type-widening is performed based on a configuration value after it has been processed by our sanitization functions. Currently, the only example that I could think of to actually exercise this distinction involves the functionality in #172 . The test I included will thus only exercise this aspect of this PR for sufficiently-old versions of omegaconf.